U.S. patent number 10,987,531 [Application Number 15/736,605] was granted by the patent office on 2021-04-27 for method for stabilizing metallic mercury.
This patent grant is currently assigned to SARP INDUSTRIES. The grantee listed for this patent is SARP INDUSTRIES. Invention is credited to Julien Borrini, Francois Hyvrard, Xavier Ibarz Formatger, Dieter Offenthaler.
United States Patent |
10,987,531 |
Hyvrard , et al. |
April 27, 2021 |
Method for stabilizing metallic mercury
Abstract
Disclosed is a method for stabilizing metallic mercury in the
form of mercury sulfide. The method includes the following steps:
a) dispersing metallic mercury in a polysulfide aqueous solution so
as to convert the metallic mercury into mercury sulfide; and b)
separating the mercury sulfide.
Inventors: |
Hyvrard; Francois (Villennes
sur Seine, FR), Borrini; Julien (Mantes-la-jolie,
FR), Offenthaler; Dieter (Heimberg, CH),
Ibarz Formatger; Xavier (Wimmis, CH) |
Applicant: |
Name |
City |
State |
Country |
Type |
SARP INDUSTRIES |
Limay |
N/A |
FR |
|
|
Assignee: |
SARP INDUSTRIES (Limay,
FR)
|
Family
ID: |
1000005513149 |
Appl.
No.: |
15/736,605 |
Filed: |
June 16, 2016 |
PCT
Filed: |
June 16, 2016 |
PCT No.: |
PCT/FR2016/051465 |
371(c)(1),(2),(4) Date: |
December 14, 2017 |
PCT
Pub. No.: |
WO2016/203162 |
PCT
Pub. Date: |
December 22, 2016 |
Prior Publication Data
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|
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Document
Identifier |
Publication Date |
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US 20180185685 A1 |
Jul 5, 2018 |
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Foreign Application Priority Data
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Jun 17, 2015 [FR] |
|
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15 55521 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C01G
13/00 (20130101); A62D 3/33 (20130101); A62D
2101/24 (20130101); A62D 2101/43 (20130101) |
Current International
Class: |
A62D
3/33 (20070101); C01G 13/00 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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41 23 907 |
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Jan 1993 |
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DE |
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Jun 2009 |
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DE |
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10 2012 102 981 |
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Oct 2012 |
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DE |
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2 072 468 |
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Jun 2009 |
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EP |
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S49-121795 |
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Nov 1974 |
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JP |
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S50-046592 |
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JP |
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S52-061196 |
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May 1977 |
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JP |
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61-033299 |
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Feb 1986 |
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JP |
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H02-034688 |
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Feb 1990 |
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JP |
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H03-010033 |
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H06-091129 |
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2006-249234 |
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JP |
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2008-208423 |
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Sep 2008 |
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JP |
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2013-503729 |
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Feb 2013 |
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JP |
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2015 147210 |
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Aug 2015 |
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JP |
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2006/016076 |
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Feb 2006 |
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WO |
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Other References
International Search Report, dated Sep. 21, 2016, from
corresponding PCT application No. PCT/FR2016/051465. cited by
applicant .
Li, "Removal of mercury from wastewater with 25 polysulfides,"
Chlor-Alkali Industry, Issue 02, pp. 75-77, Dec. 31, 1983. cited by
applicant.
|
Primary Examiner: Vanoy; Timothy C
Attorney, Agent or Firm: Nixon & Vanderhye
Claims
The invention claimed is:
1. Process for stabilizing metallic mercury in the form of mercury
sulphide, the process comprising the following steps: a) dispersion
of metallic mercury in an aqueous polysulphide solution so as to
convert metallic mercury into mercury sulphide; b) separation of
the mercury sulphide, wherein step a) is performed at a temperature
of at least 60.degree. C.
2. Process according to claim 1, in which dispersion of metallic
mercury is done using ultrasounds or a disperser.
3. Process according to claim 2, in which dispersion of metallic
mercury is done using ultrasounds.
4. Process according to claim 2, in which dispersion of metallic
mercury is done using a disperser.
5. Process according to claim 1, in which dispersion of metallic
mercury is done using ultrasounds.
6. Process according to claim 1, in which dispersion of metallic
mercury is done using a disperser.
7. Process according to claim 1, in which the polysulphide solution
has an S/Na.sub.2S molar ratio equal to 2.5 to 4.0.
8. Process according to claim 1, in which the polysulphide solution
has an S/Na.sub.2S molar ratio equal to 2.7 to 3.5.
9. Process according to claim 1, in which the polysulphide solution
has an S/Na.sub.2S molar ratio equal to 3 to 3.3.
10. Process according to claim 1, in which the active sulphur
content in the polysulphide solution is from 0.5 to 7 mol/kg.
11. Process according to claim 1, in which the active sulphur
content in the polysulphide solution is from 0.6 to 3.5 mol/kg.
12. Process according to claim 1, in which the active sulphur
content in the polysulphide solution is from 0.5 to 5 mol/kg.
13. Process according to claim 12, in which the active sulphur
content in the polysulphide solution is from 2.5 to 3.5 mol/kg.
14. Process according to claim 1, in which the S.sub.active/Hg mass
ratio is 1 to 3.
15. Process according to claim 1, in which the S.sub.active/Hg mass
ratio is 1.2 to 2.6.
16. Process according to claim 1, in which the S.sub.active/Hg mass
ratio is 1.2 to 1.5.
17. Process according to claim 1, in which the S.sub.active/Hg mass
ratio is about 1.3.
18. Process according to claim 1, further comprising after step b):
c) recovery of the polysulphide solution after the separation of
mercury sulphide in step b); d) addition of sulphur and possibly
Na.sub.2S to the polysulphide solution recovered in step c); e)
repetition of steps a) and b), using the polysulphide solution
derived from step d); f) possibly, one or several repetitions of
steps c) to e).
Description
This invention relates to a process for stabilising metallic
mercury in the form of mercury sulphide, in particular in readiness
for final or reversible storage.
Mercury has been widely used in the recent historic period for
industrial and domestic applications, chemicals (particularly
chemistry of chlorine and polymers), batteries, measurement
instruments and dental amalgams. Oil and gas extraction faces the
problem of natural pollution of some deposits by this compound that
has to be extracted and removed from hydrocarbons. Mining
extraction activities are also concerned, as are frequently illegal
placer mining activities.
The toxicity of mercury for natural environments and living
organisms is well known. Metallic or elementary mercury is toxic
for central and peripheral nervous systems. Inhalation of mercury
vapours can have noxious effects on nervous, digestive and immune
systems, and on the lungs and kidney, and can be fatal.
Neurological and behavioural disorders can be observed after
exposure to metallic mercury by inhalation. Symptoms include in
particular trembling, insomnia, memory loss, neuromuscular effects,
headaches and motor and cognitive disorders. Moderate infraclinical
signs can be observed in persons who were exposed at work to a
concentration of metallic mercury in air equal to 20 .mu.g/m3 or
more over several years. Repercussions have also been observed on
the kidneys, varying from an increase in the protein content in
urine to renal insufficiency.
Measures are taken internationally to control and limit the use of
metallic mercury and its compounds. For example, the Minamata
Convention set up as part of the United Nations Program for the
environment, calls for international action to be engaged to manage
mercury efficiently, effectively and coherently, as per Decision
25/5 adopted on 20 Feb. 2009. In Europe, there is the Regulation
(EC) No. 1102/2008 of the European Parliament and the Council of 22
Oct. 2008 on the banning of exports of metallic mercury and some
mercury compounds and mixtures and the safe storage of this
substance.
Due to these commitments, a large quantity of mercury can no longer
be recycled and reused, particular mercury derived from the
chlorine and soda industries, purification of natural gas,
extraction and melting operations of non-ferrous metals and
extraction of cinnabar ore in the European union. The quantity of
liquid metallic mercury concerned is estimated to be about 12,000
tonnes by 2020-2025.
In this context, industrial processes should be developed to
solidify and stabilise this metal in a form in which it can be
stored and manipulated in complete safety. The mercury compound
obtained can then be stored in dangerous waste storage facilities,
for example such as underground sites.
A distinction can be made between three major principles among the
different technologies used to stabilise metallic mercury: the
formation of an amalgam with another metal, incorporation of
mercury into a matrix, usually inorganic but sometimes organic, and
finally the formation of mercury sulphide.
Processes for stabilising metallic mercury based on the formation
of an amalgam with copper are described for example in U.S. Pat.
No. 6,312,499 and patent application US 2008/0234529. This type of
process requires a source of noble metal that is not good for the
global environmental balance and for the economics of the system.
The quantity of the final residue to be stored can be large.
Another disadvantage of this type of process is that amalgams,
although they are solid, have a significant vapour pressure and a
significant solubility of mercury, and thus create a problem in
terms of their stability.
The most frequently used process for treating mercury is
transformation of mercury into mercury sulphide. Mercury sulphide
is a stable form of mercury that is found in ore. Two principle
crystalline forms are observed: one, cinnabar, is red (the
vermilion pigment has been highly prized since antiquity), and the
other, metacinnabar, is black. Their crystalline structure is
different, le cinnabar is hexagonal and metacinnabar is cubic.
However, their chemical and physical properties are very similar.
The two compounds are insoluble in cold water and nitric acid and
are soluble in aqua regia.
Patent applications WO 2006/016076, DE 102012102981, DE
102007061791 and EP 2072468 describe methods of controlling the
reaction between metallic mercury and elementary sulphur. These
methods usually require complicated and energy-intensive measures
and a large excess of sulphur to achieve complete conversion.
For example, WO 2006/016076 describes a method of stabilising
metallic mercury with sulphur in the solid state in a rotary
reactor in which mercury and sulphur can be mixed non-intrusively.
Despite the fact that the mercury sulphide obtained seems to
satisfy French regulations in terms of leaching, mercury droplets
are observed in the final product in some tests and the mercury
vapour pressure in the reactor remains significant.
Patent application DE 102012102981 describes the stabilisation of
metallic mercury in the form of mercury sulphide by a liquid-liquid
reaction between metallic mercury and sulphur in the molten state
at a temperature of 120-150.degree. C. in a counter-current
reactor. In addition to the obvious energy cost of this process,
the application provides no data that can be used to assess the
performance of the transformation and the quality of the mercury
sulphide obtained.
Patent application DE102007061791 describes a process for preparing
mercury sulphide from metallic mercury and elementary sulphur. This
process is performed at high temperatures, particularly close to
the boiling temperature of mercury, advantageously combined with a
pressure slightly lower than atmospheric pressure. 18 to 44% excess
sulphur above stoichiometry is recommended to guarantee a complete
reaction of mercury.
Patent application EP2072468 A1 describes a process of transforming
metallic mercury and sulphur into mercury sulphide in the gaseous
phase. The temperature of the process is higher than 500 or even
580.degree. C., above the boiling point of mercury sulphide.
Mercury sulphide is then extracted in the gaseous phase, cooled to
50.degree. C. by sprinkling of water and then separated from
water.
In summary, all these processes make use of the reaction between
metallic mercury and sulphur, requiring complicated and
energy-intensive measures. They usually also require a large excess
quantity of sulphur to achieve complete conversion.
The use of aqueous solutions of polysulphides has also been
suggested. For example, U.S. Pat. No. 6,403,044 thus discloses a
process for stabilising metallic mercury in which the mercury or a
waste containing mercury is stabilised in several mixing steps.
Firstly, a mixture of an amalgamation agent composed of sulphur or
other products (excluding polysulphides) is mixed with the mercury
waste. This mixture is blended for a first time. An additive load
composed of microporous materials (earth, sand, cement, silica gel,
perlite, active carbon, etc.) and water is added to this first
mixture. This new mixture is then blended again. A polysulphide
solution is added to activate the reaction between mercury and the
amalgamation mixture. This mixture is blended again. Although the
solid obtained has a low leaching rate, it still contains 600 ppm
of free mercury, according to the authors. The results obtained if
there is no additive load are not as good, both in terms of
leaching rate and free mercury.
American patent U.S. Pat. No. 3,061,412 discloses a process for
preparing mercury for manufacturing of pigments. This process is
based on conversion of metallic mercury (Hg) into mercury sulphide
(HgS) by elementary sulphur in the presence of an alkaline sulphur
or polysulphide solution. Under the reaction conditions presented
in the patent and as explicitly described by the inventors,
polysulphide does not react with mercury but acts as a catalyst for
the reaction in a heterogeneous (liquid-solid) phase between
metallic mercury and elementary sulphur.
Few industrial sized installations are currently in operation
despite this large amount of work and these many patents describing
mercury stabilisation and transformation processes.
Therefore, there is still a real need for a process for stabilising
metallic mercury by which mercury is transformed into a stable
compound with high efficiencies, preferably close to total
conversion.
One purpose of this invention is thus to disclose a simple and
efficient process making use of inexpensive reagents.
It is to the merit of the inventors that they have discovered that
it is possible to transform metallic mercury into mercury sulphide
at a rate close to total conversion by using an aqueous
polysulphide solution and a means of dispersing metallic mercury in
the aqueous polysulphide solution.
Thus, one purpose of the invention is a process for stabilising
metallic mercury in the form of mercury sulphide, the process
comprising the following steps:
a) disperse metallic mercury in an aqueous polysulphide solution
making use of a dispersion means so as to convert metallic mercury
into mercury sulphide;
b) separation of the mercury sulphide obtained.
Due to the dispersion of metallic mercury in the aqueous
polysulphide solution, the process according to the invention can
simply and efficiently convert metallic mercury into mercury
sulphide. Conversion is done in a single step and the conversion
ratio is very high. In particular, it is greater than or equal to
99.8%, preferably greater than or equal to 99.9%, and even more
preferably greater than or equal to 99.99%. In one particularly
preferred embodiment, the conversion ratio is greater than or equal
to 99.999%. For comparison, the Applicant has reproduced example 1
from U.S. Pat. No. 3,061,412 and thus showed that the efficiency of
the process described in this patent based on a reaction in a
heterogeneous phase between liquid metallic mercury and solid
elementary sulphur making use of a conventional blender is
significantly lower than the efficiency of the process according to
the invention (see Example 6).
Dispersion of metallic mercury in the aqueous polysulphide solution
in step a) is done using a dispersion means. Any dispersion means
capable of dispersing mercury in the polysulphide solution and
enabling the reaction between metallic mercury and polysulphide so
as to form mercury sulphide with a conversion ratio like that
defined above can be used. For example, ultrasounds or blenders
with a high shear rate, such as dispersers, can be used.
Compared with conventional mechanical stirring, for example using a
blender, dispersing metallic mercury in the aqueous polysulphide
solution using a dispersion means has the advantage of improving
contact between mercury and polysulphide. Without wishing to be
bound by any particular theory, the inventors believe that this can
firstly accelerate the reaction rate and secondly improve the
efficiency above what is possible with conventional mechanical
stirring of the polysulphide/mercury mixture.
Thus, in one advantageous embodiment, the dispersion means are
ultrasounds. Beyond the dispersion effect, ultrasounds also perform
a function of disintegrating sulphur microballs that could
encapsulate mercury. Without wishing to be bound by any particular
theory, the inventors believe that this disintegration effect
further improves the conversion efficiency.
In another advantageous embodiment, the dispersion means is a
blender with a high shear rate, particularly a disperser.
Advantageously, this blender is operated at a shear speed (or
peripheral speed) of 9 m/s to 24 m/s, and preferably from 14 m/s to
17 m/s.
Independently of what dispersion means is chosen, step a) is
advantageously performed at a temperature higher than or equal to
60.degree. C., preferably from 60.degree. C. to 90.degree. C., more
preferably from 70.degree. C. to 90.degree. C. and even more
preferably from 80.degree. C. to 90.degree. C. Converting metallic
mercury into mercury sulphide at high temperatures greater than or
equal to 60.degree. C. actually accelerates the reaction rate and
is conducive to the formation of mercury sulphide in the form of
cinnabar (red mercury sulphide). Lower temperatures are conducive
to the formation of mercury sulphide in the form of metacinnabar
(black mercury sulphide).
Thus, the dispersion of metallic mercury in the polysulphide
solution at high temperature contributes to complete or
quasi-complete conversion of metallic mercury to mercury sulphide.
A high temperature is also conducive to the formation of cinnabar
that has the advantage of being thermodynamically more stable than
metacinnabar.
Unlike some processes according to prior art, particularly
processes that operate dry at high temperature with powder or
molten sulphur, temperatures qualified as being high within the
framework of this invention remain moderate. Treatment temperatures
in the process according to the invention and control of the
conversion to an aqueous medium thus limit risks of diffusion of
mercury vapour and inflammation.
The polysulphide solution used in step a) is advantageously a
solution of an alkali metal or alkali earth polysulphide, for
example such as sodium or potassium polysulphide. This solution can
be prepared by any method known to an expert in the subject. It can
be produced by dissolving sulphur in a solution of an alkali or
alkali earth sulphide, for example of sodium, potassium or calcium,
and preferably sodium. It can also be prepared by dissolution of
sulphur in an aqueous solution of an alkali or alkali earth
hydroxide, such as soda, potash, or an aqueous solution of calcium
hydroxide, preferably in soda.
The polysulphide solution advantageously has an S/Na.sub.2S molar
ratio equal to 2.5 to 4.0, preferably from 2.7 to 3.5 and more
preferably from 3 to 3.3.
The active sulphur content in the polysulphide solution is
advantageously from 0.5 to 7 mol/kg, preferably from 0.5 to 5
mol/kg, and more preferably from 0.6 to 3.5 mol/kg. In one
particular embodiment, the active sulphur content is 0.6 to 1.4
mol/kg, for example about 1 mol/kg. In another particular
embodiment, the active sulphur content is 2.5 to 3.5 mol/kg, for
example about 3 mol/kg.
"Active sulphur" (S.sub.active) means sulphur in the zero oxidation
state that can react with mercury in an oxidation-reduction
reaction to form mercury sulphide.
The metallic mercury used in step a) can originate from different
sources. In particular it can originate from dismantling of
installations such as units for chlorine production, for
distillation of mercury from soiled materials, for purification of
hydrocarbons extracted from some natural deposits or from any other
process for separation and extraction of metallic mercury. Mercury
can also be derived from batteries, measurement instruments, dental
amalgams. The process according to the invention can be applied
even if the metallic mercury contains water, fine sediments or
other inorganic pollution such as heavy metals.
In the meaning used in this invention, "metallic mercury" means
elementary mercury (Hg) in oxidation state 0.
In one aspect of this invention, the S.sub.active/Hg mass ratio is
1 to 3, for example from 1.2 to 2.6, preferably from 1.2 to 1.5,
and even more preferably about 1.3. Surprisingly and unexpectedly,
the inventors have observed that excess sulphur from 1.1 to 1.5,
particularly about 1.2 to about 1.3 gives rise the formation of
cinnabar (.alpha.-HgS) while higher values of excess sulphur, for
example 2.6 or more, lead to the formation of metacinnabar
(.beta.-HgS).
The Hg/polysulphide solution mass ratio is advantageously from 0.2
to 0.6. In one particularly advantageous embodiment, the
Hg/polysulphide solution mass ratio is about 0.6 and the active
sulphur content of the polysulphide solution is about 3 mol/kg. It
is thus possible to perform the installation process in a
relatively restricted reaction volume and thus make the treatment
installation more compact.
The mercury sulphide obtained in step a) can be separated using any
adapted separation means. For example, separation may be done by
filtration, settlement, centrifuging. In one embodiment, separation
is done using a filter press.
The volatile mercury content in the mercury sulphide obtained from
the process according to the invention is less than the limiting
occupational exposure limit that is 50 .mu.g/m.sup.3 in France, and
even less than 10 .mu.g/m.sup.3, according to the washing flask
test described in example 5. For comparison, the volatile mercury
content is 800 .mu.g/m.sup.3 for mercury sulphide obtained
according to example 1 in U.S. Pat. No. 3,061,412.
The process according to the invention also has the advantage that
it optimises the residual quantity of mercury to be stored because
there is no excess reagent in the recovered mercury sulphide.
The used polysulphide solution remaining after separation of
mercury sulphide in step b) can be reused in the process according
to the invention. This reuse of polysulphide has an economic and
ecological advantage because it maximises the consumption of active
sulphur and minimises the release of effluents that have to be
treated.
Therefore, according to one advantageous embodiment, the process
according to the invention includes the following steps, after step
b):
c) recovery of the polysulphide solution after the separation of
mercury sulphide in step b);
d) addition of sulphur and possibly Na.sub.2S to the polysulphide
solution recovered in step c);
e) repetition of steps a) and b), using the polysulphide solution
derived from step d);
f) possibly, one or several repetitions of steps c) to e).
According to this particular embodiment, at least one and
preferably several recyclings of the polysulphide solution are
made. During these recycling operations, the addition of sulphur to
the polysulphide solution in step d) keeps the active sulphur
content in the polysulphide solution constant. Advantageously, the
molar ratio between added sulphur and the metallic mercury to be
treated (Sadded/Hg) is about 1. According to one variant, Na.sub.2S
is also added into the recycled polysulphide solution. Each
recycling cycle is then performed with essentially the same active
sulphur content as in the initial cycle.
The process according to the invention, with or without recycling
of the polysulphide solution, is advantageously done in batches so
that the production of mercury sulphide can be started and stopped
at any time depending on needs.
The invention also relates to a treatment installation for
implementation of the mercury stabilisation process in the form of
mercury sulphide, said installation comprising: a reactor capable
of holding an aqueous solution of polysulphide and metallic mercury
and equipped with at least one dispersion means to disperse
metallic mercury in the polysulphide solution so as to convert
metallic mercury into mercury sulphide, and means of separation of
the mercury sulphide formed.
The dispersion and separation means that can be used are as
described above with reference to the mercury stabilisation process
according to the invention. When a disperser is used, it will
advantageously be installed in the bottom of the reactor to take
account of the high density of metallic mercury. In this
embodiment, the reactor is preferably also equipped with additional
stirring means configured to assure that the suspension is
homogeneous.
Separation means that can be used are as described above with
reference to the mercury stabilisation process according to the
invention. A filter press will be used in one preferred
embodiment.
In one embodiment, the reactor is also equipped with heating means
so that the conversion from mercury to mercury sulphide can be made
at high temperature, particularly at a temperature higher than or
equal to 60.degree. C., preferably from 60.degree. C. to 90.degree.
C., more preferably from 70.degree. C. to 90.degree. C. and even
more preferably from 80.degree. C. to 90.degree. C.
The installation may also include storage means such as a storage
tank for the used polysulphide solution. These storage means are
advantageously connected to the reactor so that the used
polysulphide solution can be recycled.
The invention is described in more detail below, through the
following examples that are in no way limitative but are given
solely as examples.
EXAMPLES
Preparation of Polysulphide Solutions
The polysulphide solutions used in the following examples have been
manufactured by dissolution of flower of sulphur in a sodium
sulphide solution.
The sodium sulphide solution was prepared from technical quality
flaky sodium sulphide. This compound contains 60% Na.sub.2S.
The precise compositions of the different polysulphide solutions
used are given in Table 1 below.
TABLE-US-00001 TABLE 1 Pure Pure Na.sub.2S/S c(S.sub.active) Ex-
Water Na.sub.2S Na.sub.2S S (moles/ S.sub.active (mol/ ample (g)
(g) (moles) S (g) (moles) moles) (moles) kg) 1 68.6 1.24 0.02 1.57
0.05 3.08 0.03 0.46 2 1000 22.10 0.28 29.92 0.94 3.30 0.65 0.61 3
1000 44.20 0.57 59.84 1.87 3.30 1.30 1.15 3bis 1000 68.00 0.87
92.06 2.88 3.30 2.01 1.66 4 1000 44.40 0.57 54.65 1.71 3.00 1.14
1.01 5 300 53.28 0.68 65.58 2.05 3.00 1.37 3.01
During preparation of the polysulphide solution, some of the sodium
sulphide used reacts with some of the added sulphur in a secondary
dismutation reaction to form thiosulfate. The remaining sulphur in
the zero oxidation state is active sulphur. In the framework of
this invention, it is considered that the quantity in moles of
active sulphur in the polysulphide solution is equal to the
quantity in moles of sulphur used minus the equivalent in moles of
sulphide added into the solution.
Example 1: Classical Mechanical Stirring
2.07 g of technical Na.sub.2S (namely 1.24 g of pure Na.sub.2S),
1.57 g of sulphur and 68.6 g of water were mixed in a 250 ml
beaker. The S/Na.sub.2S molar ratio of the resulting polysulphide
solution was 3 and the content of active sulphur was 0.46
mol/kg.
5 mg of metallic mercury was added after the dissolution of
sulphur.
The mixture was stirred by a magnetised bar rotating at a speed of
300 rpm. The formation of a black deposit of mercury sulphide was
quickly observed. No more metallic mercury could be distinguished
in a visual examination after 4 h30 of reaction. The conversion
efficiency is more than 90%. 1/17 g of sulphur was then added.
The reaction rate reduces over time. Thus, after 24 hours of
stirring, even with excess sulphur equal to 2.6 times
stoichiometry, the conversion efficiency does not exceed 99.5%.
This relatively low efficiency appears to be due to encapsulation
of mercury in mercury sulphide microspheres. This mercury then
becomes difficult to access under conventional stirring
conditions.
Example 2: Use of an Ultrasound Generator Probe (S.sub.active/Hg
Mass Ratio of 2.6)
For this example, an ultrasound generator probe (L250 mm--20
kHz--300 W) made by Sinaptec composed of a NexTgen ultrasound
generator at a frequency of 20 kHz was used, with a radial effect
probe for uniform diffusion of ultrasound over its entire
height.
A polysulphide solution with a content of active sulphur equal to
0.61 mol/kg and an S/Na.sub.2S molar ratio equal to 3.3 was
prepared by adding 36.8 g of technical Na.sub.2S (namely 22.1 g of
pure Na.sub.2S) and 29.9 g of sulphur in one litre of demineralised
water.
17.5 g of metallic mercury was mixed in 350 mL of this polysulphide
solution (S.sub.active/Hg molar ratio equal to 2.6). The mixture
was placed in a 400 ml Erlenmeyer flask. The probe was held in
place by a bracket and was immersed in the solution. The reaction
started at ambient temperature. The temperature increased to reach
80.degree. C. during the test. Water was added regularly to
compensate for evaporation and to stabilise the temperature.
9 recycling cycles were made after the first treatment cycle. The
quantity of mercury added in all recycling cycles was identical to
the quantity added in the first cycle (17.5 g of Hg for 350 mL of
the cycle n-1 filtrate). The results are summarised in Table 2
below.
TABLE-US-00002 TABLE 2 Cycle Added Added conversion No. S (g)
Na.sub.2S (g) Initial pH Time Solid % 1 0 0 na 2 h Black 99.9995 2
2.9 0 na 2 h Black 99.9999 3 2.91 0 11.9 2 h Black 99.9999 4 2.91 0
na 2 h Black 99.9165 5.sup.a 2.91 0 12 2 h Black 99.9304 6.sup.a
2.92 0 12 2 h 30 Black 99.9524 7.sup.a 2.92 2.15 11 2 h Black
99.9991 8 2.89 2.17 11.8 2 h Black 99.9999 9 2.89 2.14 12 1 h 30
Brown 99.9994 10 2.32 0 na 2 h Red 99.9999 .sup.aNaOH added to
adjust pH
These tests show that a dispersion of mercury by ultrasound can
considerably increase the conversion ratio of metallic mercury into
mercury sulphide. During recycling, the supplementary addition of
sulphide improves the conversion ratio. The last cycle (cycle No.
10) was done with less sulphur than the previous cycles (80% of the
theoretical value). This test showed that reducing excess sulphur
relative to mercury orients the reaction towards the red
.alpha.-HgS form, while a larger excess is conducive to the
formation of black .beta.-HgS.
Example 3: Use of an Ultrasound Generator Probe (S.sub.active/Hg
Mass Ratio of 1.3)
The test in example 2 was repeated with a polysulphide solution
obtained by mixing 73.7 g of technical Na.sub.2S (namely 44.2 g of
pure Na.sub.2S) and 59.8 g of sulphur in 1 litre of demineralised
water. The active sulphur content was 1.15 mol/kg and the
S/Na.sub.2S molar ratio was 3.3. Excess sulphur above stoichiometry
was fixed at 1.3. The quantity of mercury engaged was 70.2 g per
350 mL of polysulphide solution.
4 recycling cycles were made after the first treatment cycle. The
quantity of mercury added in all recycling cycles was identical to
the quantity added in the first cycle (70.2 g of Hg for 350 mL of
the cycle n-1 filtrate). The results are summarised in Table 3
below.
TABLE-US-00003 TABLE 3 Cycle Added Added Na.sub.2S Initial
Conversion No. S (g) (g) pH Time Solid % 1 0 0 na 2 h Red 99.9999 2
11.3 0 12.5 2 h Red 99.9675 3 11.3 0 12.5 2 h 30 Red 99.8904 4 11.3
2.14 12.5 2 h Red 99.9999 5 11.1 1.32 na 2 h Brown 99.9628
The results are comparable to those in example 2. The production of
the red .alpha.-HgS form is confirmed for an S.sub.active/Hg
ratio=1.3. Addition of complementary sulphur during recycling
cycles improves the conversion ratio. The drop in the conversion
efficiency for cycle No. 5 despite the addition of sulphur is due
to the dismutation of sulphur (secondary reaction giving rise to
the formation of thiosulfate) induced by this addition of
complementary sulphur. This phenomenon that reduces the available
quantity of active sulphur can be counteracted by increasing the
quantity of added sulphur.
Another test (Example 3bis) was performed with a polysulphide
solution prepared using 113.3 g of technical Na.sub.2S (namely 68 g
of pure Na.sub.2S), 92.1 g of sulphur and 1 litre of demineralised
water. The active sulphur content was then 1.66M and the
S/Na.sub.2S molar ratio was 3.3. The S.sub.active/Hg mass ratio was
fixed at 1.3, namely 105 g of mercury for 350 mL of polysulphide
solution. The treated mercury quantity was thus about 30% of the
mass of the polysulphide solution. After 2 h with the ultrasound
probe, the product obtained is red and the conversion efficiency is
99.999%.
Example 4: Use of a Disperser (Content of Active Sulphur 1M,
S.sub.active/Hg Mass Ratio of 1.2)
For this example, an Ultra-Turrax T18 laboratory disperser made by
IKA equipped with a 19G (19 mm diameter) tool was used. The
rotation speed is adjustable between 3000 and 25000 rpm. Note that
friction increases the temperature of the medium at high speeds
higher than about 17 000 rpm.
A series of 20 tests with recycling of the solution was carried out
under the following conditions:
The polysulphide solution was prepared with 74 g of technical
Na.sub.2S (44.4 g of pure Na.sub.2S), 54.6 g of sulphur and 1 litre
of demineralised water.
The active sulphur content of the polysulphide solution was 1.01
mol/kg and the S/Na.sub.2S molar ratio was 3.
The quantity of mercury treated per cycle was 190 g, giving an
S.sub.active/Hg mass ratio=1.2
Stirring speed 15 000 to 18 000 rpm
Reaction time: 1 h50
Start temperature: 80.degree. C.
One litre of polysulphide solution was placed in a conical flask.
The disperser was held by a bracket and immersed in the mixture. At
the end of the indicated reaction time, the mixture was filtered
using a Buchner filter. The cake was recovered by filtration and
kept without being dried for future analyses.
The results are summarised in Table 4 below.
TABLE-US-00004 TABLE 4 Mer- NaOH in 50% Conver- cury Na.sub.2S S
solution sion (g) (g) (g) (g) (%) Cycle 1 (initial) 190 74 54.7
99.99995 for 1000 ml of water Cycle 2 190 7.5 30.3 10 99.99958
Cycle 3 190 7.5 30.3 99.99991 Cycle 4 190 7.5 30.3 99.99998 Cycle 5
190 0 36 10 99.99998 Cycle 6 190 0 30.3 99.98740 Cycle 7 190 7.5 32
99.99995 Cycle 8 190 7.5 32 99.99883 Cycle 9 190 0 30.3 99.99999
Cycle 10 190 7.5 32.6 99.99935 Cycle 11 190 0 30.3 6 99.99998 Cycle
12 190 0 30.3 6 99.99920 Cycle 13 190 3.75 31 99.99997 Cycle 14 190
3.75 31 99.99998 Cycle 15 190 3.75 31 99.99994 Cycle 16 190 3.75 31
99.99999 Cycle 17 190 3.75 31 99.99999 Cycle 18 190 3.75 31
99.99998 Cycle 19 190 3.75 31 99.99999 Cycle 20 190 3.75 32
99.99999 Total 3800 149 648.4 32 average 99.9992
Not all the tests were done under the same conditions. Cycles 1 to
6 were done in a square metallic bath, the other tests were done in
a 1 litre Erlenmeyer flask. For cycles 1 to 12, the addition of
sulphur and the compensation for dismutation of sulphur were not
systematic. The solution was initially heated to 80.degree. C. but
the temperature was not kept constant throughout the reaction. The
stirring speed was constant at 18 000 rpm
Starting from cycle 13, the procedure was more systematic with the
constant addition of sulphur corresponding to 5% of the initial
quantity and compensation for dismutation of sulphur. The disperser
speed was set to 18 000 rpm for the first 15 minutes and then to
1500 rpm. A conversion from the metacinnabar form to the cinnabar
form was observed after 50 minutes' reaction time. The mercury
sulphide obtained is then constant quality with a conversion ratio
of more than 99.9999%.
A leaching test according to standard NF EN 12457-2 made with this
same average sample gives a value of leachable mercury equal to 1.1
mg/kg. If the leachate is filtered to a threshold of 0.2 .mu.m
(instead of 0.45 .mu.m according to the standard), the value is 0.1
mg/kg. This indicates that the measured mercury is not soluble
mercury but is composed of fine HgS particles.
Example 5: Use of a Disperser (Content of Active Sulphur 3M,
S/Na.sub.2S Molar Ratio=3.01, S.sub.active/Hg Mass Ratio of
1,2)
The polysulphide solution was made with 88.8 g of technical
Na.sub.2S (namely 53.3 g of pure Na.sub.2S), 65.6 g of sulphur and
300 ml of demineralised water. 228.4 g of metallic mercury was
added.
The tests were done using the same protocol as for example 4,
except for the flask volume that was 500 ml instead of one litre.
The initial temperature was 50 to 60.degree. C. because the
reaction is significantly more exothermic than with a polysulphide
solution with an active sulphur content equal to 1 M. The
temperature reached 80 to 90.degree. C. after about 10 minutes'
reaction.
An analysis of volatile mercury was made. The Mercury Tracker 3000
made by Mercury Instrument was used for this test, carrying out a
"washing flask test". For this test, a 100 g sample of the solid
obtained is placed in a closed one-litre washing flask for 24
hours. The outlet tube is then connected to the gas analyser and
the value is noted after allowing to stabilise for a few
minutes.
Three successive cycles were carried out recycling the reaction
solution. The results are given in Table 5.
TABLE-US-00005 TABLE 5 Volatile Leaching Hg (test in NF EN washing
Mercury Na.sub.2S S Conversion 12457-2 flask) (g) (g) (g) (%)
(mg/kg) (.mu.g/m.sup.3) Cycle 1 (initial) for 228.4 88.8 65.6
99.99997 0.7* 1 300 ml of water Cycle 2 228.4 8.9 38.5 99.99994
0.07 8 Cycle 3 228.4 8.9 38.5 99.99997 0.1 1 *0.2 mg/kg with
filtration to 0.2 .mu.m
The change from the metacinnabar form to the cinnabar form occurs
after 10 to 20 minutes instead of 50 minutes in the case of a
polysulphide solution with an active sulphur content of 1 M. For
cycle 3, the HgS cake was not rinsed during filtration. It will be
noted that this does not reduce the product quality. On the
contrary, a lack of very fine particles during filtration was
observed after the leaching test. The quality of the products
obtained is equivalent to or better than that obtained previously
with an efficiency of more than 99.9999%, leaching of mercury
between 0.07 and 0.7 mg/kg and a concentration of volatile mercury
equal to 1 to 8 .mu.g/m3.
Example 6: Transformation of Hg to HgS According to Example 1 in
U.S. Pat. No. 3,061,412
Example 1 in U.S. Pat. No. 3,061,412 was repeated with the same
initial product quantities and the same mixing duration using a
blender made by Kenwood.RTM. equivalent to the blender made by
Waring.RTM. used in the patent. An exothermic reaction is observed
and a very thick black sludge is recovered. This sludge was
filtered and rinsed with water.
The conversion efficiency is 94.45%. The concentration of volatile
mercury in the gas blanket of a one-litre flask containing 100 g of
residue, measured with a Mercury Tracker 3000 analyser by placing
the probe above the flask, is 800 .mu.g. This measurement method
gives lower values than the washing flask test used in Example
5.
* * * * *